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Medium

448. Numbers with prime set bits in a given range using Sieve of Eratosthenes Algorithm

Objective: Given a range, find all the numbers in the range which has prime set bits using Sieve of Eratosthenes Algorithm.

Earlier we had seen a similar problem -Numbers with prime set bits in a given range solved in the naive approach. in this algorithm, we will improve the time complexity using the Sieve of Eratosthenes.  

Example:

L = 4, R = 10
Output: 5
Explanation:
4 = 1 0 0 ( not prime)
5 = 1 0 1 (prime)
6 = 1 1 0 (prime)
7 = 1 1 1 (prime)
8 = 1 0 0 0 ( not prime)
9 = 1 0 0 1 (prime)
10 = 1 1 0 0 (prime)
Total 5 numbers which has prime set bits.

Approach:

Please read the Sieve of Eratosthenes Algorithm before continue reading.

  • Let's say the given range is L and R.
  • Find all the prime numbers from 1 to R (higher limit) using Sieve of Eratosthenes Algorithm and store it in the list.
  • Now iterate through L to R and for each integer, count the number of set bits and check if that count is present in the list created in step 2. print it. 

Output:

Numbers found for which set bits count is prime:
5 6 7 9 10

Reference: https://www.geeksforgeeks.org/prime-number-of-set-bits-in-binary-representation-set-2/

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